JavaScript新的Function表达式和新的Function构造函数

Question

My question is regarding the resulting object from using new function(){ ... } over new Function();

My understanding so far/assumption

• When we create a function using Function() or new Function() we get a object configured as a function (it's internal slots indicate a function type object)
• With or without the new operator, the Function() constructor returns a new function object
• Using a function expression returns a function object which internally uses the Function() constructor
• A function expression provides optimization with parsing the function body

My question

Following my assumption above, why does new Function(); and new function(){ ... } return different things?

The first returns a function object, but the latter returns a standard object. Given that a function expression under the hood uses the Function() constructor, why does the latter not behave the same as new Function(); ?

Using new function(){ ...} I would expect a function object, not a standard object.

Answer 1

The following lines are essentially the same:

function foo(...) {...}
var foo = function (...) {...};
var foo = new Function (...);

They all declare a new function (which is an object, that is an instance of Function , which inherits the Object prototype, ...) in the current scope which can be accessed via the foo variable.

The new keyword instantiates an object from a function which allows you to create something similar to a class instance in a standard OOP language.

Continuing the example from above:

var bar = new foo(...)

would instantiate an instance of the foo function in the current scope which could be accessed via the bar variable.

why does new Function(); and new function(){ ... } return different things?

---

I'm going to rephrase the example slightly. Instead of:

new Function()
//and
new function(){ ... }

I'm going to use

new Function()
//and
var foo = function () { ... };
new foo();

---

The reason that new Function() returns a different thing than new foo() is entirely because Function is a different function from foo .

I expect that you're not confused by the fact that new Object() returns a different object with different features than new Array() . The exact same thing is happening here.

Given that a function expression under the hood uses the Function() constructor, why does the latter not behave the same as new Function(); ?

under the hood the later expression is essentially:

new (new Function ( ... ))

Answer 2

new Function(...) returns a new function , while new function() {...} returns a new Object .

Basically, Function is just a way to evaluate a piece of code.

Objects are a way, to store data in javascript.

Side note: you can technically do

new (new Function(...))

wich returns a new object.